Checking hasNextInt() in if and then receive an input

So a simple program would be:

import java.util.*;
public class practice {
      static Scanner reader = new Scanner(System.in);
      public static void main(String[] args) {

           if(reader.hasNextInt()){

               int numberEntered = reader.nextInt();
           }


      }

}

So I have a misunderstanding. hasNextInt() is supposed to check if the next input will be an int or not. I saw this program and I don't understand how the number can be inputed. Because already for getting an input the reader.hasNextInt() needs to be true and the program hasn't got an input. So how will the program get inside the if statement?